title: The accuracy of the use of functional hip motions on localization of the center of the hip.
abstract: BACKGROUND
  The hip joint is generally considered a ball-and-socket joint, the center of which is used as an anatomic landmark in functional analyses and by surgical navigation systems. The location of the hip center has been estimated using functional techniques using various limb motions. However, it is not clear which specific motions best predicted the functional center.
  
  
  OBJECTIVE
  This study aims to compare the predicted functional center of the hip evaluated from multiplanar circumduction and star motions, and to compare this functional center with the geometric center.
  
  
  METHODS
  Eight hips in four fresh-frozen cadavers were used and verified as morphologically normal in CT scans. Three-dimensional motion of each lower limb was recorded using arrays of reflective markers rigidly attached to the femur and pelvis. Each hip was manipulated to produce circumduction or star motion, i.e., abduction-adduction and flexion extension. The hip was then dissected and the bearing surface traced with a probe, from which a best-fit sphere was calculated. The functional center was calculated from the motion data and compared to the geometric technique.
  
  
  RESULTS
  There was no difference between the functional hip center predicted by circumduction or star motions, although this was offset from the geometric hip center by up to 14 mm. For all except two hips, the functional center was less than 6 mm from the geometric hip in each anatomic direction. Test-retest differences were smaller for circumduction than for star motions.
  
  
  CONCLUSIONS
  Estimation of the hip center based on motion of the femur relative to the pelvis could localize the geometric center of the joint within 14 mm and circumduction motions were more repeatable.
  
  
  CONCLUSIONS
  Many surgical navigation systems make use of the functional hip center as a landmark for alignment or reconstruction. Errors associated with this would have a very minor influence in lower limb alignment, e.g., for knee reconstruction, but could affect proximal femoral geometry relevant to hip reconstruction.
authors.affiliation: Orthopaedic Biomechanics Laboratory, CHEO Research Institute I, Rm 252, 401 Smyth Road, Ottawa, ON K1H 8L1 Canada.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Speirs
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Benoit
authors.forename: Mélanie L
authors.initials: ML
authors.lastname: Beaulieu
authors.forename: Mario
authors.initials: M
authors.lastname: Lamontagne
authors.forename: Paul E
authors.initials: PE
authors.lastname: Beaulé
keywords: accuracy; estimation; functional center; hip center; surgical navigation